The present invention relates to a weighted transducer applied to a surface acoustic wave filter.
In general, an SAW (Surface Acoustic Wave) filter is an element used for obtaining filter characteristics as follows. An IDT (interdigital transducer) which is obtained by properly weighting an excited intensity therein is arranged in a piezoelectric substrate surface, and an SAW is transmitted and received by the IDT. As a characteristic feature of the SAW filter, amplitude and phase characteristics can be arbitrarily and independently designed.
As a conventional weighted transducer used for a surface acoustic wave filter of this type, an apodized method is mainly used. As shown in FIG. 2A, according to the apodized method, overlap widths between transducer fingers 21 and 22 are locally changed in proportion to a weighting function. Note that reference numerals 23 and 24 denote bus bars to which the transducer fingers 22 and 21 are respectively connected.
As another arrangement of the weighted transducer used for the surface acoustic wave filter of this type, a withdrawal method is used in the same manner as described above. As shown in FIG. 3A, according to the withdrawal method, although an overlap width W between transducer fingers 31 and 32 is constant, the density of the transducer fingers 31 and 32 having the overlap width W is proportional to a weighting function. Note that reference numerals 33 and 34 denote bus bars to which the transducer fingers 32 and 31 are respectively connected.
FIGS. 2B and 3B show a surface acoustic wave energy distribution 25 in the apodized method and a surface acoustic wave energy distribution 35 in the withdrawal method, respectively. In FIGS. 2B and 3B, reference symbols L represent distances from the bus bars 24 and 34, respectively.
However, the above weighted transducers have drawbacks respectively inherent thereto and have been selectively used depending on applications. That is, in the apodized method shown in FIG. 2A and 2B, a weighting function can be faithfully represented by the overlap width W, and the filter characteristics of a portion having a small weighting function, i.e., a portion having a narrow overlap width W are easily degraded by an error caused by a diffraction effect. The energy distribution 25 excited from the transducer is not uniform due to the transverse distribution of weighting functions and causes a weighting loss.
In addition, in the withdrawal method shown in FIGS. 3A and 3B, although the excited energy distribution 35 is uniform, since weighting functions are expressed by a change in density caused by the presence/absence of the overlap width W between the transducer fingers 31 and 32 connected to the bus bars 34 and 33, respectively, a quantization error is larger than that of the apodized method, and desired characteristics cannot easily be obtained.